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The Harm of Generator Loss of Magnetic Field

Mar. 09, 2019

1. Causes of generator loss of excitation

There are many reasons to cause the generator to lose excitation. Generally, in coaxial excitation system, the excitation circuit is broken (rotor circuit break, excitation circuit is broken, excitation winding is broken, etc.), and the automatic de-excitation switch miscollides or misses the brake, and so on. The magnetic field rheostat has bad contact, which makes the excitation circuit open, as well as the short circuit of the rotor circuit and the mechanical disconnection between the exciter and the prime motor in the connection to the wheel, which results in the loss of magnetic field. In the semiconductor static excitation system of large-capacity generator, the loss of excitation is often caused by the damage of thyristor rectifier components and the failure of transistor excitation regulator.

The Harm of Generator Loss of Magnetic Field.jpg

2. Influence of generator loss of magnetic field on generator itself


① After the generator loses excitation, the output reactive power becomes absorbed reactive power, and the larger the slip difference, the smaller the equivalent reactance of the generator and the larger the absorbed reactive power, which results in the overcurrent of the stator winding of the magnetized generator.


② The slip frequency current will be induced on the rotor surface (including body, slot wedge, guard ring, etc.), which will cause local overheating of the rotor, which will be the greatest harm to the generator when the rotor speed and the rotating magnetic speed of the stator are combined into a slip speed and a rotating magnetic field, and the rotor surface (including body, slot wedge, guard ring, etc.)


③ In asynchronous operation, the torque changes periodically, and the stator, rotor and its foundation are constantly impacted by abnormal mechanical torque, and the vibration of the unit is aggravated, which threatens the safe operation of the generator.


 When the degree of loss of magnetic field is serious, the generator and steam turbine rotor will be overspeed immediately if the protection is not operated in time, and the consequences will be unthinkable.


3. Influence of generator loss of magnetic field on power system

 When a generator is demagnetized, due to the voltage drop, other generators in the power system will increase their reactive output under the action of the automatic adjustment of the excitation device, thereby causing overcurrent of some generators, transformers or lines. The backup protection may be mistaken due to overcurrent, which will widen the scope of the accident.


 Low-excitation and de-energized generators absorb reactive power from the system, causing the voltage of the power system to decrease. If the reactive power reserve in the power system is insufficient, the voltage in some adjacent points in the power system will be lower than the allowable value. It destroys the stable operation between the load and each power supply, and even collapses the power system voltage and collapses.


 After a generator is demagnetized, due to the swing of the generator's active power and the drop in system voltage, it may cause an out-of-step between the adjacent normal-running generator and the system, or between the various parts of the power system, so that the system Oscillation occurred.


 The larger the rated capacity of the generator, the greater the reactive power shortage during low excitation and demagnetization, and the smaller the capacity of the power system, the smaller the ability to compensate for this reactive power deficiency. Therefore, the greater the ratio of the unit capacity of the generator to the total capacity of the power system, the more serious the adverse effect on the power system.


4. Generator loss of magnetic protection principle

 Low voltage criterion

In order to avoid the system voltage collapse caused by the loss of the generator and pose a threat to the safety of the plant power, a low voltage criterion is set.


Generally, the voltage is taken from the three-phase voltage of the auto-voltage busbar, and the three-phase voltage of the generator terminal can also be selected. Three-phase simultaneous low voltage criterion: UppPzd


After the loss of magnetism causes the generator to lose synchronization, the output power of the generator fluctuates within a certain range, and P takes the average value in one oscillation period.


 Rotor side criterion

Rotor low voltage criterion: Ur<Urlzd


In the event of a loss of magnetism, if Ur suddenly drops to zero or a negative value, the excitation low voltage criterion acts quickly (before the generator actually reaches the static limit). The loss of magnetic failure will cause the unit to lose the step. After the out-of-step, Ur and the output power of the generator fluctuate greatly. Usually, the excitation low voltage criterion will periodically operate and return. Therefore, after the excitation voltage component is out of step (enter statically) Boundary) Delayed return.


In general, the impedance setting boundary is a statically stable boundary circle.

Rotor low voltage action equation


Where: Vfd - rotor voltage

Vfl.dz - rotor low voltage action value

Vfdo - generator no-load rotor voltage

SN - rated power of the generator

Kf - rotor low voltage coefficient

P——Gene output

Pt - generator reaction power


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